The present invention relates to wireless transmission of media data, and in particular to real time adjustment of transmissions from a wireless webcam which maps the USB protocol over WiFi (802.11 protocol).
Video and audio communications provide some unique challenges in wireless communications. Unlike some other control or data signals, such as those transmitted by mice, keyboards, etc., audio data has strong periodic timing requirements while video data requires large bandwidth. For instance, 30 frames per second of video is often needed in order to preserve required quality.
Many common network protocols in use today are asynchronous and packet based. One of the most popular is Ethernet or IEEE 802.3 which is a wired networking protocol. This type of network is optimized for bursts of packetized information with dynamic bandwidth requirements that can be settled on-demand. This type of network works well for many data intensive applications in computer networks but is not ideal for situations requiring consistent delivery of time-critical data over an error-prone channel such as the air. Wireless communications often use buffers to store data such that it can be retransmitted if the wireless channel prevents its successful transmission. Retrying failed packets make it difficult to deal with isochronous data, in particular real-time streaming of synchronized audio and video data where strict latency is required.
Such video and audio data is commonly transmitted in various communications such as Video Instant Messaging, video conferencing, and so on. When data is transmitted wirelessly, in order to prevent packets from being dropped, a packet retry rate is established. The packet retry rate typically provides the maximum number of times the system retries to send a packet until it is successfully transmitted. If the retry rate is exceeded and the packet is still not transmitted successfully, the system does not try to send the packet through anymore, and the packet is dropped.
A Cerfified wireless USB (CWUSB) promoter group was formed in 2004 to define a WUSB specification. The specification sets forth a hub and spoke model, with a host controller initiating data transfers. Like wired USB, each transfer consists of 3 packets, token, data and handshake. In CWUSB, multiple token information is combined in a single packet. For isochronous transfers (e.g., streaming audio, video), bandwidth is reserved for a certain number of retries each service period and CWUSB peripherals embed enough internal buffers to allow for the max retry count.
There can be quality tradeoffs for bandwidth in USB connections. Logitech U.S. Pat. No. 6,618,776 adjusting to available bandwidth over USB by a number of mechanisms, such as varying the video frame rate, resolution, compression, etc. US Published Patent Application No. 2006/0193380 of VIXS describes managed degradation of a video stream, such as by varying compression or resolution. This management is done with a state machine in a server, which monitors the network bandwidth.